Synthesis and Characterization of Bay Region Halohydrins Derived from Benzo[a]pyrene Diol Epoxide and Their Role as Intermediates in Halide-Catalyzed Cis Adduct Formation

Abstract

The bay region epoxide of benzo[a]pyrene (anti-BPDE) alkylates DNA to form adducts with >98% trans stereochemistry. Halide ions catalyze this reaction; however, this pathway is characterized by the formation of adducts with altered cis stereochemistry. Bay region halohydrins are possible intermediates in these reactions, but are too unstable to be isolated from aqueous solutions. However, we successfully synthesized halohydrins in tetrahydrofuran (THF) by treatment of anti-BPDE with the corresponding lithium halide salt in the presence of acetic acid. Absorbance and CD spectroscopy clearly indicated the formation of chloro-, bromo-, and iodohydrins. The structure and stereochemistry of the chlorohydrin was established by NMR. Chloride addition is exclusively at the benzylic position of the epoxide, and the stereochemistry of the C-9 and -10 positions is trans. The long-wavelength absorbance band in the chloro-, bromo-, and iodohydrin is red-shifted 7, 13, and 22 nm, respectively, relative to the hydrolysis product of anti-BPDE. The ellipticity of the same absorbance band in CD spectra of enantiomerically pure halohydrins is opposite in sign compared to that of the corresponding anti-BPDE enantiomer. The relative stability of these derivatives is chlorohydrin > bromohydrin > iodohydrin. The chloro- and bromohydrins were isolated, but the iodohydrin decomposed during this manipulation. The addition of 500 mM chloride decreased the hydrolysis rate of the chlorohydrin 4-fold in 50% THF/buffer. Direct evidence for the transient formation of the iodohydrin in aqueous buffer/acetone mixtures was obtained by absorbance spectroscopy. At 1 M chloride, bromide, and iodide, alkylation of deoxyadenosine by anti-BPDE in aqueous buffer yields 85, 91, and 92% cis adducts, respectively. In the absence of halide, alkylation of deoxyadenosine in buffer by anti-BPDE, the chlorohydrin, and the bromohydrin yields 32, 65, and 83% cis adducts, respectively.

Chloride Ion Catalyzed Conformational Inversion of Carbocation Intermediates in the Hydrolysis of a Benzo[a]pyrene 7,8-Diol 9,10-Epoxide

“…To explain the different tetrol product ratios formed from the reaction of diol epoxide 1 in solutions containing chloride ion, it was proposed that a trans chlorohydrin 5 is formed as a reactive intermediate from the reaction of carbocation 2 with chloride ion, and that 5 hydrolyzes to form cis and trans tetrols in a ratio different than that formed from the reaction of 1 in the absence of chloride ion (Scheme 1). This proposed mechanism was confirmed by the independent synthesis of chlorohydrin 5 and a study of its hydrolysis reactions. , Chlorohydrin 5 is highly reactive toward hydrolysis and yields tetrols in the same ratio as that formed from the reaction of diol epoxide 1 in solutions containing sufficient concentrations of chloride ion.

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Benzo[a]pyrene Diol Epoxide Forms Covalent Adducts with Deoxycytidylic Acid by Alkylation at Both Exocyclic Amino N4 and Ring Imino N-3 Positions

“…Sample chromatograms for adducts from (+)- and (−)- anti -BPDE are shown in Figure . Previous work in our lab has shown that the relative and absolute yields of cis anti -BPDE adducts are increased by the addition of halide to the reaction mixture ( − ) or by increasing the temperature of the reaction (). The effects of chloride ion and temperature on adduct yield are shown in Figures and , respectively.…”

“…Previous work in our lab has shown that the formation of cis anti -BPDE products is catalyzed by halide ions ( − ). The halide effect arises from partial conversion of BPDE into a trans halohydrin.…”

“…This reaction pathway competes with the S N 1 pathway by which BPDE alkylates targets in the absence of halide catalysis. We have synthesized the putative chlorohydrin intermediate of the chloride-catalyzed pathway (13) and shown that in the case of dAdo alkylation it generates twice the proportion of cis products as BPDE in the absence of salt (65 vs 35%) but the same proportion of cis adducts as BPDE at high levels of chloride (88%) (12). The relative catalytic potency of halides follows the order I -> Br -> Cl -.…”

“…We have demonstrated that halide ions catalyze the alkylation of DNA by anti -BPDE and that halohydrins are intermediates in this process ( − ). Chlorohydrins are formed at physiological concentrations of salt and generate the same exocyclic amino group adducts as epoxides.…”